I. Field
The following description relates generally to wireless communications, and more particularly to uplink pilot multiplexing.
II. Background
Wireless communication systems are widely deployed to provide various types of communication; for instance, voice and/or data may be provided via such wireless communication systems. A typical wireless communication system, or network, can provide multiple users access to one or more shared resources. For instance, these systems may be multiple-access systems capable of supporting communication with multiple users by sharing the available system resources (e.g., bandwidth and transmit power). Examples of such multiple-access systems include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) systems, and orthogonal frequency division multiple access (OFDMA) systems.
Generally, a wireless multiple-access communication system can support simultaneous communication for multiple wireless terminals. Each terminal communicates with one or more base stations via transmissions on the forward and reverse links. The forward link (or downlink (DL)) refers to the communication link from the base stations to the terminals, and the reverse link (or uplink (UL)) refers to the communication link from the terminals to the base stations. Such communication links can be established via a single-in-single-out, multiple-in-single-out or a multiple-in-multiple-out (MIMO) system.
A MIMO system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, which are also referred to as spatial channels, where NS≧min{NT, NR}. Each of the NS independent channels corresponds to a dimension. The MIMO system can provide improved performance (e.g., higher throughput and/or greater reliability) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized.
A MIMO system can support time division duplex (TDD) and frequency division duplex (FDD) systems. In a TDD system, the forward and reverse link transmissions are on the same frequency region so that the reciprocity principle allows the estimation of the forward link channel from the reverse link channel. This enables the access point to extract transmit beamforming gain on the forward link when multiple antennas are available at the access point. In addition, a MIMO system may support one or more users having a plurality of transmit and/or receive antennas (e.g., single-user MIMO (SU-MIMO)) or multiple users spatially separated to support space-division multiple access (SDMA) or multiple-user MIMO (MU-MIMO).
One problem in connection with SDMA or SU-MIMO is that when multiple wireless terminals or multiple streams from a single wireless terminal are multiplexed on the same bandwidth allocation in SDMA or SU-MIMO respectively, the structure of the respective reference signals, e.g., pilot channel (PICH), should be orthogonal to each other to improve channel estimation and suppress other wireless terminals' interference using a minimum mean square error (MMSE) receiver. It is also desired that low peak-to-average ratio (PAR) is preserved by maintaining a single carrier waveform on the pilot channel to achieve improved wireless transmit power efficiency. This is especially important for improved mobile device battery performance.
For example, in single-carrier communication systems, pilot symbols are transmitted in addition to data symbols in order to provide a reference for the receiver to estimate the channel condition and accordingly demodulate the received signal. Single carrier frequency division multiple access (SC-FDMA) techniques provide an advantage over conventional OFDMA techniques in that the SC-FDMA signal has lower peak-to-average power ratio (PAPR) because of its inherent single carrier structure. As a result, SC-FDMA is especially attractive for use in uplink communications where lower PAPR greatly benefits the wireless terminal in terms of transmit power efficiency.
However, conventional uplink pilot allocation schemes result in fixed or symmetric pilot structures that inflexibly allocates pilot channel bandwidth. As a result, adaptive pilot structures are desired that maintain single carrier structure while preserving the benefits of pilot orthogonality.